Reducing metal powders



Patented Dec. 9, 1952 REDUCING METAL POWDERS Harold C. Miller, New York, N. Y., assignor to Charles Hardy, Inc., New ,York,,N.,Y,,.a.co rporation of Delaware :No'lDrawing. -Application'July 13g1948, Serial No. 38.567

.-such1-a process without substantialsintering or agglomeration; of the --powder.

ilarious powdered metals, usually reduced toa "particleksize of "50 mesh or less, are used for producing articlesof many shapes and sizesby'com- Dacting-or molding'the powder under pressure and ithensintering the molded powder to produce .fusion of"the particles and form a solid product. :E'orZthis purpose, as'wellas others. it is highly desirable' toemploy metal powders which are as freeffromzthe oxides of the metalas possible. If the surfaces of the powdered metal particles are loxidized toiany appreciable extent, such oxide is *very:'detrimental in producing satisfactory sintered molded compacts. Infact, oxide on the surfaces of the'metal particles can even result :in the production of sintered compacts which entirely lack adequate cohesion-and strength.

"Many-metal powders, and some more than others, are easily oxidized. Grinding the powder tosreduce the particle size for example, frequent- :lyresults in'increasing substantially the amount :ofgoxides in the powder.

Variousapproaches to this problem have been tried, and limited success has been achieved in reducing the oxides in the powder after the powder is compacted under pressure and While it isibeingsintered. Obviously, however, this procedureis .not suitableifor the production of dense solid products.

Heating metal powders in a reducing atmosphere has also been tried but if a temperature is employed high enough to reduce the oxides in a relativelyshort time, partial sintering of the pow- .derresultsand-when the powder is regroundto restore ,the original fine subdivision, the metal powderis rte-oxidized. Also, if a residue of carbon, ash or other matter is left on the powder after it is reduced, this residue'can also interfere materially with obtaining the proper cohesion-andstrength in a molded sintered product.

One object of myinvention is to provide a process *ofredueing or annealing metal powders byheatingthepowd'er in a reducing atmosphere under conditions which produce a reduction of the metalroxjdespresent but ,do'not produce any substantial V sintering .or agglomeration of the ppwderedparticles which would require subsew ntrind n .Anoth r object; of thetinvention .is to prevent n t ;nt ;a, :Sinterins .o a lomeration .of the -powdered particles during-afreducing heattreatment-by incorporating in-ithe powder a-smallproportion of avolatile high boilingpoint substance that is stable, does -,not'react chemically with the powder, or leave ans-objectionable-residueeafter ,heating.

A further object of "theinvention is =toselect the amount-of the substance incorporated inthe powder before heating. bytaking into account its volatility and the time and temperature eof heating so that the substance is distilled-off throughout theheatingtreatment without leaving any objectionable -:residue :in the' brightened'vor treated powder.

Other objects andadvantages ofmy invention will be explained or will loe'apparent from'zthe toll-owing description thereof.

,1 have found-that oxidized metal powdersoi varioustypes, both ferrousand'non ferrous, can be satisfactorily annealed or :brightened to produce a loose, finely divided-productrelativelylow or free-from oxides bymixing with'the powder a-small amount ofa substance that has a relatively high boiling point but is completely volatile at the temperature of heat treatment. that is stableenough so it will not clecomp'ose'at the temperature employed to form'elemental carbon, and thatis unreactive chemically with the metal of the powderxtreated. This mixture is then heated 'at an'appropriatetemperature for asuitable time in a reducing atmosphere. Apparently such a substance, if properlycs'elected and used in the proper amount, is continuously distilled off through :the powder xduring the reducing heat treatment, and'its'volatilization serves'to maintain the powder'in loosened form by an action that is more mechanical than chemical in character, but which prevents substantial sintering or agglomeration of'the particles during the heating and by keeping the powder looseassists to some extent in reducing the oxides present.

The substance selectedwill naturally vary with different metalpowderswhich'are heated to different temperatures, and'the amount of the-substance needed'will vary depending upon its volatility and the time'and temperature of lieat'irig. It is mostdesirableto provide enough of "thematerial so that the last of it is distilled ofi at approximately "the end of the heating treatment. Of course, if a slight residue of the substance is left, the powder can'b'e-r'eheated fora -ft-zwni' utes, or the heating continued for a fewminutes longer. On the other hand, if the heating --is continued for any-substantial-length 6ftime after the last 0f "the substance --is distilled off, the

powder settles and packs, and is apt to become part all sintered.

The time and temperature of the reducing heat treatment are not particularly critical, although it is desirable to avoid heating the powders to a true sintering temperature, and, on the other hand, it is desirable as a practical matter to employ a hi h enough tem erature for the particular powder treated to reduce the oxides effectively within a relatively short time, such as 30 minutes or less. For example, plain copper powders may be satisfactorily reduced by heating at a temperature of the order of 600 C. for 30 minutes or less. may be used for a shorter time and somewhat lower temperatures may also be employed, as will be understood by those skilled in the art.

Bronze and brass powders can be satisfactorily reduced using the same temperatures, although with some alloys heating may have to be continued for a somewhat longer period such as 45 minutes in order to obtain complete reduction. With other alloys of copper, such as copper-lead powders, somewhat lower temperatures of the order of 400 C. are more suitable in order to avoid any sweating out of the lead. On the other hand, with ferrous powders, such as a plain iron powder, a reducing temperature of the order of 800 C. is usually desirable, and satisfactory reduction can be accomplished with the powder at this temperature for a period of 15 minutes. These times and temperatures are given simply by way of illustration and are not intended as a limitation on my process.

I also prefer to carry out the reducing heat treatment with the powder spread out in a relatively thin layer in shallow trays or boats, or distributed on a belt moving through a furnace. Layers from a fraction of an inch to three inches or more in depth may be treated successfully according to my invention.

The reduction of the metal oxides may be obtained by maintaining in the furnace an atmosphere of suitable reducing gas, preferably one that contains a high proportion of hydrogen.

Pure hydrogen is preferable, although such gas as disassociated ammonia gas or cracked illuminating gas may also be employed. It is also desirable, of course, to maintain the reducing atmosphere around the powder while it is cooling from the highest temperature.

The substance added to the metal powder may be either a solid or a liquid, and I have found that high boiling point hydrocarbons or mixtures of such hydrocarbons are particularly satisfactory for this purpose. Other high boiling point organic compounds than hydrocarbons may be employed, if they have the proper volatility, do not react with the metal, and do not decompose under the conditions of heating to leave a deposit of carbon or other objectionable material in the final product. For example, I have found that various mineral oils are suitable for purposes of my invention, and waxes such as paraffin, can also be used. Polybutene has also been found satisfactory. Either single compounds or mixtures of compounds may be employed. On the other hand, turpentine and other vegetable oils carbonize at a relatively low temperature and are not suitable. Compounds that decompose during heating to form completely volatile constituents without leaving any ash or other deposits may be used.

The proportion of the substance employed will vary depending upon various factors 511 1 as he Somewhat higher temperatures volatility of the substance used, the temperature and time of heating to be employed, and to some extent on the depth of the layer of powder subjected to treatment. As a general proposition, I have found that from about 0.3% to 2% is satisfactory for most purposes, although my invention is not limited to this particular range.

The process may be applied to metal powders in various conditions and may even be applied to powders fresh from the electrolytic bath which are still wet with water and electrolyte. In this case, however, it is usually desirable to employ a slightly higher proportion of the substance to be added since part of it is carried off by generation of steam as the wet powder is heated up.

The substance added to the powder may be simply stirred into the powder by any suitable form though it is usually desirable in the case of a very highly oxidized powder to subject it successively to two separate reducing treatments with appropriate addition of volatile substances before each treatment. This is desirable because of the length of time required for good reduction at a single heating, and because the volatile substance may be distilled off completely some time before the, end point is reached if the'heating period is too long.

The following examples are given as illustrative of the process of my invention and show clearly the ease with which reduction can be accomplished by this procedure without producing sintering or agglomeration of the powder such as to require regrinding.

Example I An oxidized copper powder having a screen analysis of --30 mesh with over 40% of the powder -325 mesh was mixed with 0.5% by weight of 3 in 1 oil, spread out'in a tray in a layer 0.25 inch deep and heated in a hydrogen atmosphere at 600 C. for 30 minutes. Upon cool ing, the powder had' a bright copper color, was practically free from oxides and still analyzed m6Sh with 39.6% 325 mesh.

A sample of the same powder heated without the oil addition but with all of the other conditions the same, was agglomerated to such an extent that considerable attrition was necessary to break it up into a fine powder.

Example II.The same powder was mixed with #10 W motor oil and heated in a layer 1.25 inches deep at 600 C. for 5 minutes. It was found that under these conditions an addition of 0.3% by weight of the oil produced a product that could be easily broken up by shaking on a screen, and that an' addition of 0.5% produced a final product which was completely free and loose after heating.

Example III.A sample of the same powder was heated under the conditions of Example I but mixed with 0.4% by weight of polybutene instead of the oil. The results were equally good.

Example 1V.The procedure of Example II was carried out by substituting paraffin chips for the oil and it was found that 1% by weight of the paramn was needed to produce as good results as were obtained with 0.3% of the motor oil.

Example V.An 80-10-10 bronze powder having a screen analysis similar to the copper powder in the ioregoingexamples was mixed with ..Q.5%rby weight of f3,,in l,.o il and heated in a treatment, the powder=was-shaken on a screen andjound to be satisfactorygfor immediate :use.

..A:samp1e::of the same .poivdersheated .nnderithe same conditions but .without -.the oil addition agglomerated or sintere-d into a hard mass that required regrinding.

ExampZe VL A cop-per ---coated lead powder fresh from the chemical deposition bath was mixed with 0.5% by weight of 3 in 1 oil and heated in a 0.25 inch layer in a hydrogen atmosphere at a temperature of 00 C. for 30 minutes. Because ofthe very high oxygen content of the original powder (around 6%) this process was repeated a second time. At the end of this second treatment, the powder was a bright copper color and in a completely, satisfactory loose finely divided state.

Example VII.Iron powder was successfully reduced to a bright gray powder by mixing it with 1% of #10W motor oil and heating it in hydrogen in a layer 1.25 inches deep at 800 C. for minutes. The same powder heated under the same conditions without the oil sintered or agglomerated into a hard mass that had to be ground to reform it into a powder.

It will be apparent to those skilled in the art from the foregoing explanation and examples that my process can be carried out economically and simply with standard equipment to produce powders free from or extremely low in oxygen content and yet retaining completely their powdered form.

The procedure may be carried out in batches or continuously and the volatile substance may be added in any convenient manner to obtain at least a fair degree of distribution. If the substance added is a solid or a viscous liquid, it may be diluted or dissolved in a volatile solvent before it is added.

The terms and expressions which I have employed are used as terms of description and not of limitation, and I have no intention, in the use of such terms and expressions, of excluding any equivalents of the features shown and described or portions thereof, but recognize that various modifications are possible within the scope of the invention claimed.

I claim:

1. A process of annealing or brightening oxidized metal powder without substantial sintering thereof which comprises mixing with the powder a small proportion of a high boiling point, completely volatile, chemically stable substance, and heating the mixture in a reducing atmosphere to reduce metal oxides in the powder, the proportion of said substance being selected in accordance with the time and temperature of heating so that the substance is distilled off throughout the reducing treatment without leaving any appreciable residue in the brightened powder.

2. A process of annealing or brightening oxidized metal powder without substantial sintering thereof which comprises mixing with the powder a small proportion of a high boiling point, completely volatile, stable hydrocarbon, and heating the mixture in the form of a relatively thin layer in a reducing atmosphere to reduce the metal oxides in the powder, the proportion of said hydrocarbon used being selected in accordance with the time and temperature of heating so that the hydrocarbon is substantially completely distilled ofi throughout the reducing treatment.

'6 1.3. Inazprocessofheating ametal powder in a reducing atmosphere to brighten oreanneal the powder and reduce metaloxides therein, the step of adding to the powder-a high boil-ingpoint substance capableoof gcompletevolatilization at the temperatureato which the powder is heated. in an amount which issufficient'tosupply products of volatilization in the powder throughout'substantially the: entire: period of "heating.

.4. A process of brightening -:oxidize'd-metal powder composed principally :of copper which comprises adding to the powder-a small proportionofamineraloil that volatilizes without leaving a residue, and heating the powder and oil in an atmosphere containing hydrogen as a major constituent at a reducing temperature for said powder within the range of about 350 to 650 C. for a sumcient time to reduce oxides in the power, the proportion of said oil added being sufficient to supply products of volatilization in the powder throughout substantially the entire period of heating.

5. A process of brightening oxidized metal powder composed principally of copper which comprises adding to the powder a small proportion of polybutene, and heating the mixture in an atmosphere containing hydrogen as a major constituent at a reducing temperature for said powder within the range of about 350 to 650 C. for a sufficient time to reduce oxides in the powder, the proportion of said polybutene added being sufficient to supply products of volatilization in the powder throughout substantially the entire period of heating.

6. A process of brightening oxidized metal powder composed principally of copper which comprises adding to the powder a small proportion of paraffin, and heating the mixture in an atmosphere containing hydrogen as a major constituent at a reducing temperature for said powder within the range of about 350 to 650 C. for a sufficient time to reduce oxides in the powder, the proportion of said paraffin added being sufficient to supply products of volatilization in the powder throughout substantially the entire period of heating.

7. A process of heating a metal powder composed principally of iron in a reducing atmosphere composed principally of hydrogen which comprises adding to the powder a small proportion of a high boiling point volatile hydrocarbon, and heating the mixture at a temperature of about 700 to 850 C. for a sufiicient time to reduce oxides in the powder, the proportion of said hydrocarbon added being suiiicient to supply products of volatilization in the powder throughout substantially the entire period of heating.

8. A process as defined in claim 4 in which the proportion of mineral oil is from about 0.3% to 1.0% of the weight of the original powder.

9. A process as defined in claim 5 in which the proportion of polybutene is from about 0.4% to 1.5% of the weight of the original powder.

10. A process as defined in claim 6 in which the proportion of parafiin added is from about 0.6% to 2.0% of the weight of the powder.

11. A process as defined in claim 7 in which the proportion of hydrocarbon added is from about 0.8% to 2.0% of the weight of the powder.

12. A process of brightening oxidized copper powder without substantial sintering thereof which comprises mixing a small amount of a completely volatile hydrocarbon oil with said powder, spreading the powder out in a relatively thin layer, and heating the layer in a reduc- "8 REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,905,809 Cowan Apr. 25, 1933 FOREIGN PATENTS 10 Number Country Date 152,956 Switzerland Feb. 29, 1932 

1. A PROCESS OF ANNEALILNG OR BRIGHTENING OXIDIZED METAL POWDER WITHOUT SUBSTANTIAL SINTERING THEREOF WHICH COMPRISES MIXING WITH THE POWDER A SMALL PROPORTION OF A HIGH BOILING POINT, COMPLETELY VOLATILE, CHEMICALLY STABLE SUBSTANCE, AND HEATING THE MIXTURE IN A REDUCING ATMOSPHERE TO REDUCE METAL OXIDES IN THE POWDER, THE PROPORTION OF SAID SUBSTANCE BEING SELECTED IN ACCORDANCE WITH THE TIME AND TEMPERATURE OF HEATING SO THAT THE SUBSTANCE IS DISTILLED OFF THROUGHOUT THE REDUCING TREATMENT WITHOUT LEAVING ANY APPRECIABLE RESIDUE IN THE BRIGHTENED POWDER. 